Technical Field
The present disclosure relates to bulk acoustic wave resonators, currently designated as BAW resonators in the art. It is more specifically directed to a method for manufacturing BAW resonators in which a compensation layer, capable of guaranteeing a temperature-stable behavior of its operating frequency, is provided.
Description of the Related Art
A BAW resonator comprises a resonant core, or piezoelectric resonator, formed of two electrodes between which is arranged a layer of a piezoelectric material. When an electric field is applied to the piezoelectric layer by application of a potential difference between electrodes, this results in a mechanical disturbance in the form of an acoustic wave. This wave propagates within the BAW resonator. The fundamental resonance establishes when the acoustic wavelength in the piezoelectric material substantially corresponds to twice the thickness of the piezoelectric layer. Schematically, a BAW resonator behaves as an on switch at the resonance frequency and as an off switch at a so-called antiresonance frequency.
BAW resonators are currently formed above a semiconductor substrate, for example, on a silicon wafer. An acoustic isolation device is then provided between the resonant core and the substrate to avoid a leakage of the acoustic waves into the substrate. There mainly exist two types of BAW resonators: BAW resonators suspended on a membrane and BAW resonators isolated from the substrate by a Bragg mirror.
Suspended BAW resonators, better known as FBARs (Film Bulk Acoustic Wave Resonators) or TFRs (Thin Film Resonators), comprise an isolating air layer between the resonant core and the substrate. Thus, a cavity is provided in the substrate or an air bridge is provided above the substrate. Such resonators have the disadvantage of being difficult to form due to the mechanical fragility of such a device.
BAW resonators with a Bragg mirror, better known as SMRs (Solidly Mounted Resonators), are isolated from the substrate by a reflector, currently a Bragg mirror. They have a stronger structure, better adapted to standard manufacturing methods in microelectronics.
BAW resonators with a Bragg mirror are considered herein.
FIG. 1 is a cross-section view schematically showing a BAW resonator 1 with a Bragg mirror formed on a semiconductor substrate 3. Although FIG. 1 shows a single resonator, in practice, many resonators are formed simultaneously on a same semiconductor wafer.
Resonator 1 comprises a piezoelectric resonator 5 formed of the stacking of a lower electrode 5a, of a layer 5b of a piezoelectric material, and of an upper electrode 5c. As an example, the piezoelectric material may be aluminum nitride (AlN), lead zirconate titanate (PZT), or zinc oxide (ZnO). Electrodes 5a and 5c may be made of molybdenum (Mo), tungsten (W), or aluminum (Al).
An isolation structure 7, for example, a Bragg mirror, forms an interface between piezoelectric resonator 5 and substrate 3. Reflector 7 is an alternated stack of layers 7a of a material with a high acoustic impedance, for example, tungsten (W), and of layers 7b of a material with a low acoustic impedance, for example, silicon oxide (SiO2). The thickness of each layer 7a, 7b is selected to be substantially equal to one quarter of the resonance acoustic wavelength in the material forming it. At the operating frequency, for example, the resonance frequency, the reflector behaves as an acoustic mirror and the waves are confined within the resonator. To obtain a good acoustic isolation, the difference in acoustic impedance between the materials forming layers 7a and 7b must be high. Further, the quality of the acoustic isolation increases along with the number of layers 7a, 7b of the alternated stack.
The methods of deposition of the different layers of resonator 1 do not provide resonance frequencies with the desired accuracy. Substantial variations of the resonance frequency can especially be observed between resonators formed on the same semiconductor wafer.
For this reason, a frequency adjustment layer 9, for example, made of silicon nitride, is provided at a surface of resonator 1. The presence of this layer 9 modifies the behavior of the resonator 1 and especially its operating frequency (for example, the resonance frequency). In a final manufacturing step, a thickness of layer 9 is adjusted by local etching, until the desired frequency is accurately obtained. As an example, an ion etching may be used.
A disadvantage of BAW resonators with a Bragg mirror of the type described in relation with FIG. 1 is the strong dependence of their resonance frequency on temperature. This results from the influence of temperature on the velocity of acoustic waves in the different layers of the resonator 1 and especially in the piezoelectric layer 5b. The temperature coefficient of frequency, or TCF, expresses, in parts per million per degree Celsius (ppm/° C.), the temperature drift of the resonator frequency. In each of the materials forming the resonator 1, the acoustic waves have a certain propagation velocity, and for each material, this propagation velocity has a certain temperature coefficient of velocity, or TCV. All the TCVs of the various materials determine the TCF of the BAW resonator. Generally, the materials of the piezoelectric layers and of the electrodes have a negative temperature coefficient of velocity (TCV). Conversely, materials such as silicon oxide have a positive temperature coefficient of velocity (TCV).
It has been suggested to provide, in the stack of layers forming the BAW resonator, at least one temperature compensation layer having a temperature coefficient of velocity (TCV) of a sign opposite to the TCV of all or of the majority of the other layers, for example, silicon oxide. It is thus attempted to decrease the temperature drift of the BAW resonator, that is, to decrease the absolute value of the TCF.
Several locations have been suggested for the temperature compensation layer, and especially between upper electrode 5c and piezoelectric layer 5b. This layer being placed in the resonance region, it has a strong influence upon the behavior of the resonator. A very small thickness, for example, from 10 to 50 nm, is thus sufficient to provide the desired thermal stabilization behavior. However, a disadvantage of such a layout is that the compensation layer has a significant influence upon the acoustic behavior. This results in a degradation of the quality factor and of the electromechanical coupling of the resonator.
It has also been suggested to provide a compensation layer between upper electrode 5c and frequency adjustment layer 9. A disadvantage of this structure is that the compensation layer makes the step of frequency adjustment by etching of layer 9 more complex. Indeed, the sensitivity of the resonator frequency to the thickness of layer 9 is modified by the presence of the compensation layer. Another disadvantage associated with the provision of this additional layer is the degradation of electric performances, and especially of the electromagnetic coupling and of the quality factor.
In addition to the above-mentioned disadvantages, the above-described temperature compensation modes have the disadvantage of being rather inaccurate. They introduce a strong TCF dispersion in BAW resonators. In particular, substantial variations of the TCF can be observed between BAW resonators manufactured from a same wafer, and all the more from different wafers.